It is known to form underbody covers or engine compartment covers from glass fiber-reinforced plastics in a forming method with high internal mold pressures. The glass-fiber reinforcement typically consists of woven mats or nonwoven mats, but also from bulk glass fibers, which are, however, as non-oriented as possible and which are introduced into a plastic matrix predominantly of polypropylene. The semifinished products made available in this way are usually plates made from glass fiber-reinforced thermoplastic (GMT) or rod-shaped granulate (LFT: long fiber thermoplastic). The rod-shaped granulate is composed of a glass fiber filament bundle of approximately 20 mm length, which is enclosed by a polypropylene shell. Before molding, the plates are heated in a furnace or the LFT granulate is melted in a plastifying unit, in order to then be placed in the open mold of the press.
Meanwhile, it is also typical to process the glass fibers in a direct draw-in process together with plastic granulate in a plastifying unit (D-LFT), without having to pass through the intermediate step of an LFT semifinished product. For increased temperature requirements, it is also typical to use as the plastic matrix a glass fiber-reinforced duroplastic material made from polyester resin, which fully cures in a heated die (SMC: sheet molded compound).
The resulting components typically have a thickness of approximately 1.5-2.5 mm and a basis weight of approximately 2 kg/m2. The maximum component size currently possible is approximately 1.0 to 1.5 m2, due to the very high molding pressures of approximately 200-300 bar and the associated high machine costs for presses with a pressing force of more than 3000 t.
New production methods allow the production of lighter-weight and larger surface-area components with significantly lower molding pressures. Here, a nonwoven mat made from glass fibers and plastic fibers, e.g., polypropylene or polyester, is created as a semifinished product and is covered on both sides with two plastic cover films, e.g., also polypropylene (LWRT: low weight reinforced thermoplastic). The core layer of this composite has the property of expanding (lofting) under heating. With this material lofted to a total thickness of approximately 10 mm, the edge region can be pressed compact (completely consolidated) through a suitable die shape, while the structure of the nonwoven core with the cover films can be maintained in the remaining region. This structure leads to very inherently stiff components with a relatively low basis weight of below 1.5 kg/m2. Because in this method the die cavity does not have to be formed by a flowing mass, significantly smaller molding pressures (approximately 10 bar) are produced and it is possible to mold with clamping surfaces of 4 m2 and more without additional measures. A disadvantage in this method is that structures for reinforcing or required for additional functionality, such as connecting pieces, NACA openings, attachment domes, etc., can be attached only to a limited extent, if at all. Newer developments in the field of LWRT have foam as the core layer and a glass fiber-reinforced PP nonwoven as the cover layer. Here, further weight reduction is possible for a comparable stiffness.
It is further known to provide these engine compartment shields and underbody covers on the side facing the engine or the exhaust installation with heat shields and sound absorbers.
Sound absorbers usually consist of coated PUR foam or coated polyester nonwoven, but also of deep-drawn chamber structures or microperforated films and plates. Typically, such sound absorption molded parts are later bonded, clipped, or fused onto the engine compartment shielding. However, it is also known to produce a complete noise enclosure, that is, a support and compartment absorber, in a blow-molding method in one processing step. However, based on the process, this creates a considerable restriction on the material selection of the support and absorber and thus also on the physical properties, especially as concerns glass-fiber reinforcement of this component and its properties with reference to stiffness, strength, and impact resistance.
Heat shields are composed of prefabricated aluminum, which is clipped on or fused on by means of a special connection layer. The fusing and shaping of fusible aluminum in the die is also known.
Recently, the combination of sound absorption and heat insulation in the form of aluminum membrane absorbers and microperforated aluminum films has also become known.
It is further known to produce wheel-well covers from nonwovens or combinations of nonwovens and films. Nonwoven variants have advantages in terms of production costs and component weight compared with injection-molded wheel well covers. In particular, it has been shown that this nonwoven has a favorable acoustic effect against noises from splashed water and stone impacts.
Recently, production has moved towards attaching nonwoven also on the road-surface side of the underbody covers and noise enclosure. Here, it has been shown that the noise from the engine, gearing, and exhaust installation is reduced by this street-side lining and then even when the underbody has already been closed completely by a noise enclosure. To take full advantage of the potential, the nonwoven thickness should be significantly greater than the current typical nonwoven thickness layers of approximately 1 mm.